1. Technical Field of the Invention
This invention pertains to a current reference circuit. In particular, this invention relates to an improved current reference circuit design, thereby providing exceptional tolerance and a standby mode which ensures circuit operation in the absence of an input frequency.
2. Background Art
Switched capacitor current reference circuits require an input frequency in order to produce an output current. This can be a problem in applications where the circuit must respond quickly at the moment the input starts switching or where the oscillator that provides the input frequency is itself powered by the current reference circuit.
Current reference circuits are widely used in microprocessors and ASICs to supply constant current for PLLs and other high speed circuits. Switched capacitor current references are recognized as having very good tolerances compared to alternative approaches since their output currents depend only on thin oxide capacitance (C.sub.1 and C.sub.2), the input frequency, and a voltage reference. However, they suffer the disadvantage that they do not supply current until the input frequency begins switching, and they rise slowly due to the filtering elements required to remove ripple which is present in switched capacitor DC generating circuits. Prior art methods include use of DC reference circuits which nearly always have poorer tolerances, and use of expensive off-chip components. As technology progresses, power supply tolerance is becoming more demanding and the frequency must be very precise in many critical applications.
It is an object of the present invention to provide a current reference with excellent tolerance and no off chip components.
It is another object of the invention to provide an optional standby mode for a current reference circuit wherein the output is maintained in the absence of an input frequency.